High Elastic Modulus Polymer Electrolytes

ABSTRACT

A polymer that combines high ionic conductivity with the structural properties required for Li electrode stability is useful as a solid phase electrolyte for high energy density, high cycle life batteries that do not suffer from failures due to side reactions and dendrite growth on the Li electrodes, and other potential applications. The polymer electrolyte includes a linear block copolymer having a conductive linear polymer block with a molecular weight of at least 5000 Daltons, a structural linear polymer block with an elastic modulus in excess of 1×10 7  Pa and an ionic conductivity of at least 1×10 −5  Scm −1 . The electrolyte is made under dry conditions to achieve the noted characteristics.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 60/744,243 filed Apr. 4, 2006, titled HIGH ELASTIC MODULUS POLYMERELECTROLYTES; and U.S. Provisional Patent Application No. 60/820,331filed Jul. 25, 2006, titled SYNTHESIS OF DRY POLYMER ELECTROLYTES; thedisclosures of which are incorporated herein by reference in theirentirety and for all purposes.

STATEMENT OF GOVERNMENTAL SUPPORT

This invention was made with government support under ContractDE-AC02-05CH11231 awarded by the United States Department of Energy toThe Regents of the University of California for the management andoperation of the Lawrence Berkeley National Laboratory. The governmenthas certain rights in this invention.

FIELD OF THE INVENTION

The invention relates to polymer electrolytes. More particularly, theinvention relates to high elastic modulus, high ionic conductivitypolymer electrolytes comprising linear block copolymers, and methods ofmaking them.

BACKGROUND OF THE INVENTION

Polymer membranes with high ionic conductivity are important forapplications such as solid-state batteries and fuel cells. Theperformance of these materials depends not only on their electricalproperties but also on other properties such as shear modulus,permeability, and the like. The mechanical properties of polymerelectrolytes are particularly important in secondary solid-state lithium(Li) batteries. One of the challenges in the field of rechargeable Liion batteries is to combine high energy density with good cyclabilityand electrode stability. Batteries that employ Li metal anodes for highenergy density applications suffer from failures due to side reactionsand dendrite growth on the Li electrodes. Repeated cycling of thebatteries causes roughening of the Li surface and eventually to dendriteformation, which reduces battery life and compromises safety.

Recent theoretical work indicates that dendrite growth can be stopped ifthe shear modulus of current polymer electrolytes can be increased bythree orders of magnitude without a significant decrease in ionicconductivity. Other studies have shown that cation transport isintimately coupled to segmental motion of the polymer chains. Thesestudies indicate that dendrite growth on the electrode surface can beprevented by introducing a highly rigid electrolyte (elastic modulus ofabout 1 GPa) between the two electrodes. This high modulus requirementessentially renders most rubbery polymer electrolytes incompatible withthe electrode material, as the elastic moduli of typical rubberypolymers are about 1 MPa. For example, poly(ethylene oxide) (PEO) melt,one of the most studied polymer electrolytes, has an elastic modulus ofless than 1 MPa. High ionic conductivity is obtained in soft polymerssuch as PEO because rapid segmental motion needed for ion transport alsodecreases the rigidity of the polymer. Glassy polymers such aspolystyrene offer very high moduli (about 3 GPa) but are poor ionconductors. Thus, conductivity and high modulus have appeared to bealmost mutually exclusive goals in polymer electrolytes.

There is, therefore, a need to develop a new methodology for decouplingthe electrical and mechanical properties of polymer electrolytematerials. Such a material would be useful as a solid phase electrolytefor high energy density, high cycle life batteries that do not sufferfrom failures due to side reactions and dendrite growth on the Lielectrodes.

SUMMARY OF THE INVENTION

The present invention provides a polymer that combines high ionicconductivity with the structural properties required for Li electrodestability. The polymer is useful as a solid phase electrolyte for highenergy density, high cycle life batteries that do not suffer fromfailures due to side reactions and dendrite growth on the Li electrodes,and other potential applications. The polymer electrolyte includes alinear block copolymer having a conductive linear polymer block with amolecular weight of at least 5000 Daltons, a structural linear polymerblock with an elastic modulus in excess of 1×10⁷ Pa and an ionicconductivity of at least 1×10⁻⁵ Scm⁻¹. The electrolyte is made under dryconditions to achieve the noted characteristics.

In specific embodiments, the linear block copolymer is a diblockcopolymer characterized by bicontinuous lamellar phases of the polymerblock constituents, and the structural polymer block can form acontinuous rigid framework through which the conductive polymer blockforms continuous nanostructured ionically conductive channels.

In one aspect, the invention relates to a polymer electrolyte. Theelectrolyte includes a linear block copolymer having a conductivepolymer block with a molecular weight of at least 5000 Daltons, astructural polymer block with an elastic modulus in excess of 1×10⁷ Pa,and an ionic conductivity of at least 1×10⁻⁵ Scm⁻¹.

In another aspect, the invention relates to a method of making a polymerelectrolyte. The method involves, in an oxygen and moisture freeenvironment, forming a linear block copolymer having a conductivepolymer block with a molecular weight of at least 5000 Daltons and astructural polymer block with an elastic modulus of at least 1×10⁷ Paand incorporating a Li salt into the linear block copolymer such thatthe resulting polymer electrolyte has an ionic conductivity of at least1×10⁻⁵ Scm⁻¹.

In a further aspect, the invention relates to battery cells comprising aLi anode, a cathode and linear block copolymer electrolyte in accordancewith the invention disposed between the anode and cathode. The cell canbe cycled without detrimental dendrite growth on the anode.

These and other aspects and advantages of the present invention aredescribed below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and constitute apart of this specification, illustrate one or more embodiments and,together with the detailed description, serve to explain principles andimplementations of the invention.

In the drawings:

FIG. 1A is an illustration of a block copolymer in accordance with thepresent invention in isolation, showing the linearity of the polymerblock chains.

FIG. 1B depicts the chemical structure of a polystyrene-b-poly(ethyleneoxide) (PS-b-PEO) diblock in accordance with one embodiment of thepresent invention.

FIGS. 2A-C illustrate embodiments of a PS-b-PEO diblock.

FIG. 3 is a graph illustrating SAXS profiles obtained from SEO/salt andSIEO/salt mixtures in accordance with the present invention.

FIG. 4 is a graph illustrating the dependence of ionic conductivity on r(Li salt per EO unit) and temperature in SEO (36-25), a block copolymerelectrolyte in accordance with the present invention.

FIGS. 5A-5B are plots comparing ionic conductivity of block copolymerelectrolytes in accordance with the present invention to the molecularweight of their respective PEO blocks.

FIG. 6 is a graph illustrating the frequency dependence of the storageand loss shear moduli of SEO (36-25), a block copolymer electrolyte inaccordance with the present invention, with and without salt and PEO.

FIG. 7 is a graph illustrating the frequency dependence of elasticmoduli for the series of copolymers referenced in FIG. 3.

FIG. 8 is a plot of the results of DC cycling of a Li/SEO/Li cell withan electrolyte in accordance with the present invention.

FIG. 9A is a bright field electron micrograph of a block copolymer inaccordance with the present invention.

FIG. 9B is a lithium energy filtered electron micrograph of the sameblock copolymer region as FIG. 9A.

FIG. 10 presents data showing lamellae thicknesses for block copolymersin accordance with the present invention obtained from variousmeasurements.

FIG. 11 is a plot of Li lamellae thickness normalized by the PEOlamellae thickness as a function of molecular weight.

FIG. 12 is a plot comparing the normalized Li lamellae thickness andnormalized conductivity versus molecular weight for various blockcopolymers in accordance with the present invention.

FIG. 13 is a simple schematic diagram of a battery cell in accordancewith the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Reference will now be made in detail to specific embodiments of theinvention. Examples of the specific embodiments are illustrated in theaccompanying drawings. While the invention will be described inconjunction with these specific embodiments, it will be understood thatit is not intended to limit the invention to such specific embodiments.On the contrary, it is intended to cover alternatives, modifications,and equivalents as may be included within the spirit and scope of theinvention as defined by the appended claims. In the followingdescription, numerous specific details are set forth in order to providea thorough understanding of the present invention. The present inventionmay be practiced without some or all of these specific details. In otherinstances, well known process operations have not been described indetail in order to not unnecessarily obscure the present invention.

It will, of course, be appreciated that in the development of any actualimplementation of the invention, numerous implementation-specificdecisions must be made in order to achieve the developer's specificgoals, such as compliance with application- and business-relatedconstraints, and that these specific goals will vary from oneimplementation to another and from one developer to another. Moreover,it will be appreciated that such a development effort might be complexand time-consuming, but would nevertheless be a routine undertaking ofengineering for those of ordinary skill in the art having the benefit ofthis disclosure.

INTRODUCTION

As noted above, the present invention provides a polymer that combineshigh ionic conductivity with the structural properties required for Lielectrode stability. The polymer is useful as a solid phase electrolytefor high energy density, high cycle life batteries that do not sufferfrom failures due to side reactions and dendrite growth on the Lielectrodes, and other potential applications. The polymer electrolyteincludes a linear block copolymer having a conductive linear polymerblock with a molecular weight of at least 5000 Daltons, a structurallinear polymer block with an elastic modulus in excess of 1×10⁷ Pa andan ionic conductivity of at least 1×10⁻⁵ Scm⁻¹. The electrolyte is madeunder dry conditions to achieve the noted characteristics.

In specific embodiments, the linear block copolymer is a diblockcopolymer characterized by bicontinuous lamellar phases of the polymerblock constituents, and the structural polymer block can form a rigidframework in which the conductive polymer block forms nanostructuredionically conductive channels.

Linear Block Copolymers

A linear block copolymer can be used to produce a polymer electrolytethat can combine high ionic conductivity with the structural propertiesrequired for Li metal electrode stability. The electrolyte is useful asa solid phase electrolyte for high energy density, high cycle lifebatteries that do not suffer from failures due to side reactions anddendrite growth on the Li electrodes. The linear block copolymer may bea composite polymer electrolyte having relatively soft (e.g., having anelastic modulus on the order of 1 MPa or less) nanoscale conductingchannels embedded in a relatively hard (e.g., having an elastic moduluson the order of 1×10⁷ Pa or more) polymer matrix that need not beionically conductive. Ionic conductivity in the channels is conferred bya Li salt incorporated with the soft polymer block. It has been foundthat particular configurations and fabrications of such linear blockcopolymers enable the combination of high ionic conductivity and highelastic modulus in a polymer electrolyte.

While prior work in the field has suggested block copolymers as suitablematerials for electrolytes, these prior block copolymers were composedof relatively soft (non-glassy) polymer blocks that were modified withvarious side chains.

The electrolytes of the present invention, however, comprise blockcopolymers with linear polymer block constituents, one of which is softand conductive to Li ions; another of which is hard and need not beionically conductive. FIG. 1A provides an illustration of such acopolymer in isolation, showing the linearity of the polymer blockchains. The inventive polymer electrolytes include a linear blockcopolymer having a conductive linear polymer block with a molecularweight of at least 5000 Daltons, a structural linear polymer block withan elastic modulus in excess of 1×10⁷ Pa and an ionic conductivity of atleast 1×10⁻⁵ Scm⁻¹.

In a specific embodiment, the copolymer is a linear diblock copolymer ofpoly(ethylene oxide) (a rubbery, linear polymer, highly ionicallyconductive when embedded with an appropriate Li salt) with polystyrene(a non-conductive, glassy, linear polymer); apolystyrene-b-poly(ethylene oxide) (PS-b-PEO) diblock, the chemicalstructure of which, is illustrated in FIG. 1B. This combination has beenfound to provide both the high ionic conductivity and high rigiditysought after for high performance solid state polymer electrolytes.

FIGS. 2A-C illustrate embodiments of a PS-b-PEO diblock in accordancewith the invention. The PS-b-PEO diblock, generally numbered 10,exhibits an extensive phase behavior, which allows for bicontinuousphases. In these embodiments, in the bicontinuous PS-b-PEO phases, themajor (e.g., greater than 50%) phase (PS) 12 provides a rigid frameworkfor the nanostructured ionically conducting channels (PEO) 14.

FIGS. 2A-C illustrate the shape and placement of polymer blockscomprising conceptual diblocks in accordance with the present invention,and are not meant to be limiting. It will be known and understood thatthe diblock may be in any shape that is necessary and the placement ofthe polymers within the diblock may vary as needed. As illustrated inthe FIG. 2A, the PEO channels 14 may be positioned in an orderlyplacement within the PS matrix 12. Alternatively, as illustrated in FIG.2B, the PEO channels 14 may be branched, such as in a gyroid phase,within the PS matrix 12. FIG. 2C illustrates another specific embodimentin which the PEO channels 18 are layered in a lamellar arrangement withthe PS matrix 16. While it has previously been believed that networkmorphologies are essential for ion transport, it was surprisingly foundthat diblock copolymers in accordance with the present invention withsuch non-network lamellar morphologies are very effective iontransporters.

In many instances, the diblock copolymer has a non-conducting phase thatis continuous, and, in most cases, the major component, which differsfrom previously proposed block copolymer electrolytes. The mechanicalproperties of such block copolymer electrolytes are dominated by thoseof the non-conducting component. This enables independent control overthe electrical and mechanical properties of the electrolyte. Thestructural linear polymer block major phase forms a rigid framework,with the conductive polymer block forming nanostructured lonicallyconductive channels through the rigid framework.

In specific embodiments, the linear block copolymer is a diblockcopolymer and may be characterized by bicontinuous phases of the polymerblock constituents such that there are continuous ionically-conductivechannels (or pathways) through a continuous rigid framework. In thisway, the beneficial properties of both polymer block materials(structural and conductive) exist throughout the material. Blockcopolymers are known in the art to self-assemble into a variety ofstructures or arrangements. As noted above, a particular arrangement fordiblock copolymers in accordance with the present invention withadvantageous properties is lamellar, although other arrangements arepossible.

An important feature of the block copolymers is the combination of softand hard polymer blocks to form a rigid material that is also highlyionically conductive. Thus, the structural polymer block has an elasticmodulus of at least 1×10⁷ Pa, and generally from about 1×10⁷ Pa to 3GPa. A suitable polymer for the structural block is polystyrene, aglassy polymer with an elastic modulus of about 3 GPa. Other mechanismsfor obtaining suitable rigid frameworks include cross-linking andcrystallization. The ionically conductive polymer, on the other hand,typically has an elastic modulus of no more than 1 MPa; poly(ethyleneoxide), a specific material for which advantageous properties have beenfound in block polymers of the present invention, has an elastic modulusof less than 1 MPa.

Other examples of polymers suitable as conductive or structural polymersfor these block copolymer electrolytes include poly vinyl pyrrolidine(conductive), poly acrylates (conductive), polyvinylcyclohexane(structural), epoxies (structural), and polypropylene (structural).

As noted above, the conductive polymer block chains of the blockcopolymer have a molecular weight of at least 5000 Daltons. This isnotable since prior work in the field has indicated an inverserelationship between the molecular weight of conductive polymers andtheir ionic conductivity. To the contrary, it has been found that theionic conductivity of the block copolymers of the present inventionincreases with increasing molecular weight of the constituent polymerblocks. Accordingly, in various embodiments, the structural polymer hasa molecular weight of at least 5000 Daltons. Further, in variousembodiments, the ionic conductivity of the block copolymer increaseswith increasing molecular weight of the conductive polymer block, so theconductive and structural polymer block chains have molecular weights ofat least 15,000 Daltons; or at least 50,000 Daltons; or up to about100,000 Daltons or more. Ionic conductivity of greater than 1×10⁻⁴ Scm⁻¹and up to about 1×10⁻³ Scm⁻¹ has been achieved with these relativelyhigh molecular weight block copolymers in accordance with the presentinvention.

As noted above, ionic conductivity is achieved for the conductivepolymer block by incorporating an appropriate Li salt into theconductive polymer block. It is important that incorporation of the Lisalt be done in a controlled environment and under conditions selectedto achieve the desired performance in the resulting polymer electrolyteproduct. This is discussed further below. Suitable Li salts include Lipentafluoromethane sulfonyl methide, lithiumbis[1,2-oxalato(2-)-O,O′]borate (LiBOB), and Li bis(trifluoro methanesulfonyl)methane, and others known in the art. One specific salt isLiN[SO₂CF₃]₂ (also known as LiTFSI). Self-assembly of the conductivepolymer block results in ionically conductive channels in the ridgedstructural polymer block framework.

Further, as also described below, it has been found that the enhancedionic conductivity obtained with increasing molecular weight of theconductive polymer block is correlated with a concentration of Li saltin a central portion of the ionically conductive channels in the blockcopolymer electrolytes.

While many specific embodiments of the invention exhibit a diblockcopolymer structure, other structures are also possible. For example, anadditional polymer block may be added to adjust the properties of theresulting copolymer, in this case a triblock copolymer. One suitabletriblock copolymer is a polystyrene-block-polyisoprene-blockpoly(ethylene oxide) (S-I-EO) triblock copolymer. For example, a S-I-EOtriblock copolymer with molecular weights of 11,000 Daltons for thepolystyrene block, 6000 Daltons for the polyisoprene block, and 9000Daltons for the poly(ethylene oxide) block was found to have a measuredconductivity range from 4×10⁻⁴ Scm⁻¹ to 9×10⁻⁴ Scm-1 in the 90-120° C.temperature range, for a molar ratio of Li ions to ethylene oxide,r=0.02.

Electrolyte Fabrication

Generally, a method of making a polymer electrolyte in accordance withthe present invention comprises; in an oxygen and moisture freeenvironment, forming a linear block copolymer having a Li ion conductivelinear polymer block with a molecular weight of at least 5000 Daltonsand a structural linear polymer block with an elastic modulus of atleast 1×10⁷ Pa; and incorporating a Li salt into the linear blockcopolymer, such that the resulting polymer electrolyte has an ionicconductivity of at least 1×10⁻⁵ Scm⁻¹. Care must be taken in themanufacture of the linear block copolymer electrolytes to preventadverse reaction of the Li salt and to obtain the polymer product withthe advantageous performance noted herein.

A block copolymer electrolyte in accordance with the present inventioncan be made by synthesizing the structural polymer block by livinganionic polymerization. Then, a monomer of the conductive polymer blockand a cryptand catalyst are added to the structural polymer block livinganionic polymerization mixture. The polymerization of the conductiveblock is then allowed to proceed. The reaction may proceed to completionand, after it is terminated with a suitable reagent, the resultingdiblock copolymer product is precipitated and freeze-dried. Other livingpolymerization methods like cationic and radical polymerizationtechniques can also be used to synthesize the block copolymers.

To render the copolymer conductive to Li ions, a Li salt is blended withthe freeze-dried diblock copolymer using a moisture-free solvent suchthat the Li salt is dissolved with the conductive polymer block. Theresulting polymer/salt solution is then freeze dried to remove thesolvent. The freeze-dried dry polymer/salt mixture can then be subjectedto heat and pressure (compression molded) to form a freestanding polymerelectrolyte film.

A detailed description of a method of making a linear diblock copolymeras applied to a specific embodiment follows. The following example willbe described using SEO; however, it will now be realized that othermaterials and preparation methods may be used. As such, the followingexample is discussed for exemplary purposes only and is not intended tobe limiting.

Synthesis of PS-b-PEO

In an exemplary embodiment of the invention, the diblock copolymer,polystyrene-b-poly(ethylene oxide) (SEO) is synthesized using livinganionic polymerization to attain good control of the molecular weightand achieve low polydispersity. All of the synthetic steps are performedon a high-vacuum line and inside an argon glove box to ensure an oxygenand moisture free environment.

The first step is to purify the solvent, benzene, over calcium hydridefor at least 12 hours at room temperature. Benzene is then distilledonto a sec-butyllithium stage for further purification.

The next step is purification of the styrene monomer, and synthesis ofpolystyrene. The styrene monomer is purified on calcium hydride (CaH₂)by stirring for a few hours to remove trace amounts of water. Themonomer is then distilled into a flask (e.g., short-neck round-bottomflask) containing dibutyl magnesium (DBM). The monomer is stirred overDBM overnight. A clean reactor, in which polymerization will take place,is placed on the high-vacuum line and flamed with a torch to removeadsorbed water and solvent. After flaming, about 20 mL of the purifiedsolvent is distilled from the sec-butyllithium stage into the cleanreactor. This is done to prevent freezing and degassing a large amountof solvent. After the addition of 20 mL of benzene, the reactor is takenoff the vacuum line, and into the glove box at which point thecalculated amount of sec-butyllithium initiator required for reaction ispipetted into the reactor.

A very good seal is maintained during transfer back onto the vacuumline. After the reactor is placed back on the line, the mixture isfrozen with liquid nitrogen and degassed 1, 2, 3, 4, or 5 times toremove trapped argon from the vessel and promote fast distillation. Theremainder of the solvent can then be distilled into the reactor. In onearrangement, enough solvent is added to yield a mixture of approximately8-10 wt % polymer in solution.

The first block, polystyrene (seen as 12 in FIGS. 2A-B, and 16 in FIG.2C) is synthesized by distilling a calculated amount of styrene from theDBM stage into a graduated ampoule and then into the reacting vessel,which contains benzene and the sec-butyllithium initiator. The reactionis carried out at room temperature for approximately 12 hours and acharacteristic yellow “living” polymer mixture is obtained uponcompletion. The mixture is stirred continuously during the reaction toensure uniform mixing of the monomer. The reactor is then taken into theglove box and an aliquot is taken out and terminated with degassedmethanol or isopropanol for characterization using gel permeationchromatography to determine its molecular weight.

This completes the synthesis of the first block, ready for the additionof the ethylene oxide (EO) block. A flask (e.g., short-neck round-bottomflask) with freshly powdered calcium hydride is placed on the vacuumline and opened up to vacuum to remove all air. The flask is maintainedat 0° C. Ethylene oxide monomer, a gas at room temperature (b.p. 10° C.)is condensed onto the calcium hydride at 0° C. and left to stirovernight to remove residual moisture. The monomer-containing flask ismaintained at a dry ice/IPA mixture temperature. The ethylene oxide iskept cold at all times during purification. The ethylene oxide/calciumhydride mixture is frozen and degassed twice before the purificationsteps.

The monomer is purified with two stages of n-butyllithium by stirringfor 30 minutes at 0° C. at each stage. 10 mL of n-butyllithiun incyclohexane is used for each stage. The purifying agent, n-butyllithium,is completely dried by pulling vacuum to get rid of the cyclohexanesolvent in which the n-butyllithium is dissolved. This can preventcontamination and ensures an accurate measurement of the amount ofethylene oxide (EO) monomer. The ethylene oxide monomer is distilledinto a graduated ampoule maintained at 0° C. after purifying at bothstages of n-butyllithium.

Following purification and distillation into a graduated ampoule, 1 mlof the EO monomer is distilled into the living polystyrene reactor foraddition of a single EO unit. Only one EO unit is added to the livingpolystyrene chain because of the strong complexation of the lithiumcation to the oxygen in the ethylene oxide unit. The Li cation shows astrong association with the ethylene oxide unit and, thus, nopropagation reaction can occur. To polymerize the second block, acryptand catalyst (e.g., tert-butyl phosphazene base (tBu-P₄)) is addedto the reaction mixture (containing living EO capped polystyrene chains)to complex the lithium ions and facilitate the polymerization ofethylene oxide. To prepare the catalyst base, the solvent in which it isdissolved is removed and a known amount of benzene distilled into theflask. This solution can then be pipetted into the reactor in theglovebox. The reactor is then reattached to the high-vacuum line anddegassed 1, 2, 3, 4, or 5 times to remove trapped argon from the vesseland promote fast distillation. The remainder of the EO is then distilledfrom the graduated ampoule into the reactor.

The reaction is allowed to proceed for three to four days at 45° C.during which the color changes to a dark blue. The diblock copolymer(SEO) is then terminated with methanol in the glove box and purified byprecipitation in cold hexane. The precipitated polymer is dissolved inbenzene and filtered through a 0.2 μm filter. The filtered polymer isthen freeze dried to remove all solvent. The freeze-dried polymer ischaracterized by gel permeation chromatography (GPC) to determine themolecular weight and by nuclear magnetic resonance (NMR) to calculatethe volume fraction of each block.

Electrolyte Preparation

Polymer electrolytes are prepared by blending the freeze-dried SEOcopolymers with the lithium salt, LiN[SO₂CF₃]₂ (LiTFSI) in a moisturefree environment. This is achieved in a few steps. A LiTFSI/THF solution(about 10% w/w) is prepared in the glove box, and stored as a stocksolution. All the copolymer samples are weighed and vacuum dried in aheated antechamber and brought into the glove box. The dry copolymersamples are then dissolved in dry benzene. To this copolymer/benzenesolution, the necessary amount of the LiTFSI stock solution is added inthe argon glove box. All solvents used in the electrolyte preparationare doubly-distilled to remove trace amounts of moisture. Thepolymer/salt solution is then freeze-dried in a glove box compatibledesiccator for one week to remove all solvent. The freeze-driedpolymer/salt mixture is loosely packed into a pellet in a die that has adiameter slightly less than the spacer inner diameter. The pellet isplaced in a Teflon™ bag along with a spacer, such that the pellet is inthe center of the spacer's slot. The spacer is made of a non-conductingreinforced resin, and can withstand high pressure and temperaturewithout deforming. The pellet is then subjected to a high pressure(about 1 kpsi) at about 100° C. This yields a clear, freestandingpolymer electrolyte film with the spacer around it. The spacer definesthe edge and the thickness (and hence the area, and the volume) of thefreestanding polymer electrolyte film, and provides a means for easytransfer in and out of the membrane-electrode assembly. All the stepsare carried out in the glove box.

Structure and Performance

While the invention is not limited by any particular theory, it isbelieved that the high elastic modulus, high ionic conductivity blockcopolymers of the present invention are correlated with a particularmorphology. It has been found that for at least some specificembodiments of the invention with advantageous properties, thebicontinuous phases of the polymer block constituents of the diblockcopolymer have a lamellar arrangement and that the Li salt segregatesitself to the ionically conductive block channels. In addition, withincreasing molecular weight of the polymer block constituents, the Lisalt increasingly concentrates itself to a central portion of theionically conductive block channels and ionic conductivity increases.

As described further below with reference to FIG. 8, the linear blockcopolymer electrolytes of the present invention exhibit good cyclingperformance. No dendrite formation was observed for 80 cycles in DCLi/polymer/Li test cells.

EXAMPLES

The following examples provide details relating to composition,fabrication and performance characteristics of block copolymerelectrolytes in accordance with the present invention. It should beunderstood the following is representative only, and that the inventionis not limited by the detail set forth in these examples.

To characterize the electrolyte, experimental data results were obtainedby methods such as transmission electron microscopy (TEM), small angleX-ray scattering (SAXS), AC impedance spectroscopy, and rheology. ACimpedance spectroscopy measurements were made using a test cell onthermo-stated pressed samples in the glove box, and a Solartron 1260Frequency Response Analyzer machine connected to a Solartron 1296Dielectric Interface. The polymer samples for TEM and SAXS were annealedusing the same thermal history as that used for conductivitymeasurements. The electrolyte samples were kept at 120° C. for 2 hrs,and cooled to room temperature. Thin sections (about 50 nm) wereprepared using an RMC Boeckeler PT XL Cryo-Ultramicrotome operating at−100° C. Imaging was done on a Zeiss LIBRA 200FE microscope operating at200 kV. SAXS measurements were made on hermitically sealed samples. Thedata were collected for various temperatures between 140° C. and roomtemperature during cooling. Rheological measurements were performedusing an ARES rheometer from Rheometric Scientific Inc. with a parallelplate geometry. Approximately 1 mm thick samples were placed between 8mm plates for SEO, and 50 mm plates for PEO, in a closed oven with aNitrogen (N₂) atmosphere. The ratio of Li ions to ethylene oxidemoieties, r, in the electrolyte was varied from 0.02 to 0.10. Unlessotherwise noted, due to the lack of qualitative changes in propertieswith salt concentration, data was obtained at r=0.02. Alternatingcurrent (AC) impedance and rheological data, obtained from mixtures of a20 kg/mol PEO homopolymer and Li[N(SO₂CF₃)₂], serve as the baseline forevaluating the properties of the composite electrolyte.

Table 1 lists the characteristics of the different kinds of copolymersthat will be discussed:

TABLE 1 Mn Mn (PS) (PEO) d spacing Copolymer g/mol g/mol φ_(EO)Morphology (nm) SEO(36-25) 36400 24800 0.38 Perforated 47.9 ± 0.8Lamellae SEO(74-98) 74000 98100 0.55 Lamellar 101.2 ± 3.4  SEO(40-54)39700 53700 0.55 Lamellar 66.4 ± 1.4 SEO(40-31) 39700 31300 0.42Perforated 44.4 ± 0.6 Lamellae SEO(16-16) 16200 16300 0.48 Lamellar 30.4± 0.3 SIEO(11-6-9) 11200 9000 0.31 Lamellar 33.0 ± 0.3

Two different block polymer types were utilized to obtain theexperimental data. The first is polystyrene-block-poly(ethylene oxide)diblock copolymers (SEO) doped with lithiumbis(trifluoromethylsulfonyl)imide, Li[N(SO₂CF₃)₂]. The PS-rich phaseprovides mechanical rigidity, while the PEO phase provides ionicconductivity. The second is apolystyrene-block-polyisoprene-block-poly(ethylene oxide) triblockcopolymer (SIEO). The presence of the polyisoprene rubbery domains isknown to increase the impact strength of polystyrene.

FIG. 3 is a graph illustrating SAXS profiles obtained from SEO/salt andSIEO/salt mixtures. The SAXS data were obtained from SEO/salt andSIEO/salt mixtures with r=0.02 and at a temperature of 90° C. Forclarification, SIEO (11-6-9) (marked A) has a ratio ofpolystyrene-polyisoprene-poly(ethylene oxide) of 11-6-9. SAXS dataobtained from pure SEO and SIEO samples were indistinguishable from thedata shown in FIG. 3. The long range order, as gauged by the sharpnessof the primary and higher order peaks, is better in low molecular weightsamples than in the high molecular weight samples. This is expected dueto the slow diffusion in strongly segregated block copolymers.

The SAXS profiles of SEO (36-25) (marked C) are similar to perforatedlamellar phases, while those from the other samples show the presence ofa lamellar phase. TEM images of SEO (36-25) (not shown for brevity)indicate that the PEO lamellae appear dark and perforated, while the PSlamellae show no evidence of perforations, in agreement with the SAXSdata shown in FIG. 3. Based on the similarity in the SAXS profilesobtained with and without salt, the addition of salt at lowconcentrations to the SEO copolymer may not affect the morphology. TheTEM and SAXS data indicate the absence of an interpenetrating networkphase in the block copolymer electrolytes.

FIG. 4 is a graph illustrating the dependence of ionic conductivity on rand temperature, respectively, in SEO (36-25). FIG. 4 illustrates thatthe conductivity has a maximum at r≈0.067, regardless of temperature.This trend is similar to that observed in PEO-salt homopolymer-basedsystems. At low salt concentrations, ionic conductivity increases withsalt concentration due to the increase in the number of charge carriers.At high salt concentrations, transient cross linking of the polymerchains and neutral ion pairs result in reduced conductivity. FIG. 4illustrates the conductivity at various temperature ranges. Itillustrates that SEO (36-25) was conductive at 90° C. and higher.

FIGS. 5A-5B are graphs comparing ionic conductivity of the compositeelectrolytes as a function of the molecular weights of the PEO blocks.FIG. 5A illustrates a systematic increase in conductivity with themolecular weight of the PEO block (M_(PEO)). The observed trend isopposite to that obtained in pure PEO/salt mixtures where theconductivity decreases with increasing molecular weight and eventuallyreaches a plateau. The conductivity, σ_(PEO), of the PEO homopolymerwith r=0.02 at 90° C. and 120° C. was measured to be 3.7×10⁻⁴ S/cm and5.6×10⁻⁴ S/cm, respectively. The conductivity obtained in the highestmolecular weight sample, SEO (74-98) (marked F in FIG. 3), is comparableto that of pure PEO. Data obtained (not shown) from SEO (74-98) at 100°C. show σ peaks at a value of about 10⁻³ S/cm at r=0.08. These values ofconductivity are adequate for some battery applications.

The PEO volume fraction in the composite electrolyte, φPEO, varied from0.38 to 0.55. If all of the PEO channels in the nanostructuredelectrolyte provided conducting pathways for the ions, and if theconductivity of the PEO channels were identical to that of PEOhomopolymers, then the value of the conductivity of a doped SEO sample,σ_(max) would be the product of φ_(PEO)σ_(PEO). The ratio σ/σ_(max) thusnormalizes the measured conductivity data for differences in φ_(PEO).FIG. 5B is a plot of σ/σ_(max) versus M_(PEO). FIG. 5B shows that theionic conductivity of the electrolytes may be mainly affected byM_(PEO), and not by block copolymer composition.

FIGS. 5A and 5B illustrate that: (1) it is possible to makeself-assembled conducting nanostructured electrolytes withnon-conducting matrix phases, and (2) the magnitude of the conductivityof such electrolytes is in the range of the theoretical upper limit thatcan be expected from such systems. The samples were not subjected to anyspecial processing steps to ensure that the PEO lamellae were connectedor aligned. Connectivity of the PEO phase occurs reproducibly byquiescent annealing at 120° C. Although it has been widely understoodthat highly connected network phases are essential for highconductivity, the above data indicates that that is not the case.

FIG. 5B illustrates the fact that σ/σ_(max) for the highest molecularweight sample is close to the theoretical maximum of 0.67 expected for astructure comprised of randomly oriented grains. It is believed thatdissociated Li ions are tightly coordinated with the ether linkages inPEO, and thus disruption of this coordination could lead to faster iontransport. The disruption of coordination may be due to two reasons: (1)the contact between the ions and the polystyrene/poly(ethylene oxide)interfaces, and (2) the deformation of PEO chains due to self-assembly.The polystyrene/poly(ethylene oxide) interfacial area (per unit volume)decreases with increasing molecular weight. As noted above withreference to FIG. 3, potential reason (1) appears incorrect. As such,ion transport in microphases may be faster than in bulk.

In contrast, it is known that block copolymer chains stretch when theyform ordered phases due to the traditional balance of energy andentropy. On average, PEO chains are stretched in the lamellae, comparedto the homopolymer. The extent of stretching depends on copolymer chainlength, N, and the Flory-Huggins interaction parameter between theblocks, χ. The range of the product χ_(N) in the SEO diblock copolymersranged from 25 to 130 at 90° C., indicating that these copolymers inaccordance with the invention are far from the weak segregation limit.Another indication of the stretched nature of the PEO chains is thescaling of the PEO lamellae thickness, d_(PEO), calculated from theperiodic length scale, d, given in Table 1 with N_(EO), the number of EOunits in PEO block. A least squares power law fit through the d versus Ndata yields d_(PEO)=0.18*N_(EO) ^(0.69) (both the prefactor and exponentwere free parameters in the fit), which is the expected result in thestrongly segregated limit. Thus, the stretched PEO chains in the highmolecular weight block copolymers are not as tightly coordinated with Liions compared to PEO random coils, which leads to enhanced conductivity.

FIG. 6 is a graph illustrating the frequency dependence of the storageand loss shear moduli. The graph shows the frequency (ω) dependence ofthe storage and loss shear moduli, G′ and G″, respectively, of pure SEO(36-25), SEO (36-25) with r=0.020, and pure PEO homopolymer. The dataobtained from the pure SEO (36-25) and SEO (36-25) with r=0.020 areindistinguishable. The frequency-independence of the moduli and the factthat G′ is an order of magnitude larger than G″ indicate that the blockcopolymer electrolytes are elastic solids. The data also indicates thatthe addition of small amounts of salt has no detrimental effect on themechanical properties of the materials. The value of G′ obtained fromthe SEO (3625) electrolyte is 100 times larger than the plateau modulusof pure PEO and 6 orders of magnitude larger than G′ of the PEOhomopolymer. The molecular weight of the PEO homopolymer is similar tothat of the PEO block in the SEO (36-25) copolymer. Thus, nanostructuredelectrolytes have roughly half the conductivity of homopolymer PEO butlarger shear moduli by several orders of magnitude.

FIG. 7 is a graph illustrating the frequency dependence of elasticmoduli for the diblock copolymers referenced in FIG. 3. The highermolecular weight copolymers that exhibit high ionic conductivity yieldelastic moduli of the order of 10⁸ Pa. By adding the PS block to the PEOchain, the shear modulus increases by several orders of magnitude whilekeeping the ionic conductivity near the level of pure PEO.

FIG. 8 plots the results of cycling a Li/SEO (74-98) r=0.02/Li cell witha 250 μm thick electrolyte at 90° C. The cell was formed with afreestanding polymer electrolyte film formed as described above herein.A spacer with the freestanding polymer electrolyte film was thensandwiched between two aluminum masks that defined the exposedelectrolyte area available for lithium (Li) deposition. This assemblywas then secured to a slotted disk. The slots in the disk ensure thatthe electrolyte surface was exposed for Li deposition. The slotted-diskwith 30 electrolyte/mask assemblies was then secured to a spindle in theLi deposition chamber. The spindle was connected to a motor that canrotate the spindle, along with the slotted plate with electrolyte/maskassemblies. Once the slotted plate was secured to the spindle, Lipellets were placed in the crucible of the vapor-deposition chamber. Thedeposition chamber was then closed and left overnight under a very highvacuum (<10⁻³ mbar). The high vacuum ensures that the electrolyte and Lipellets are free of any adsorbed solvent molecules that may react withLi vapors. When the pressure in the deposition chamber was below 10⁻³mbar, the crucible was heated to ˜200° C., and Li was deposited from Livapors onto the exposed surface of the electrolyte. The thickness of thedeposited Li layer/film was monitored, and vapor deposition was stoppedonce a desired (˜1 μm) Li-layer thickness was achieved. Then thedeposition chamber was opened and the slotted plate was flipped andsecured back to the spindle, exposing the other face of the freestandingelectrolyte film that had not yet received any deposited Li. Thedeposition chamber was closed and the process of Li-deposition wasrepeated. This yields symmetric Li/SEO/Li cells for performing DCcycling measurements. In each cycle, a 50 μA/cm² current density wasapplied to the cell for 2 h, followed by a rest period of 1 h, followedby a 50 μA/cm² current density applied in the opposite direction. Thevoltage required to achieve this current density was independent of timeover 80 cycles, indicating the lack of dendrite growth during theseexperiments.

The structure of the block copolymer electrolytes of the presentinvention was investigated to determine if a structural correlation forthe observed properties could be determined. Poly(styrene-block-ethyleneoxide) (SEO) copolymer electrolytes were prepared as described above.Samples were pressed at 120° C. into 0.1-1 mm thick disks using amechanical press. All sample preparation was done without exposing thematerials to atmospheric water or oxygen.

Electron microscopy images were analyzed to determine the width of thelithium and poly(ethylene oxide) channels. Micrographs were segmentedinto boxes that were approximately two times the periodicity of thestructures. The minimum periodicity, d_(min), was taken to be the truespacing of the nanostructured electrolyte. Regions which show aperiodicity within 10% of d_(min) were selected for furtherquantification. The intensity distribution of these images wasnormalized to maximize the dynamic range of contrast. A high-pass filterwhich removes low-frequency data below a radius of 30 pixels was used toremove some of the noise from the images. The image was thresholded,such that pixels are either black or white, to measure systematicallythe lamellae thicknesses. The channel thicknesses were measured by handevery 0.04 d_(min) along the lamellae normal.

FIG. 9A is a bright field image of SEO (16-16)/salt mixture (r=0.085).The amorphous nature of the polymer is clearly visible. FIG. 9B is thelithium energy filtered micrograph of the same region as FIG. 9A. Thelight regions correspond to the presence of lithium, and the darkregions to the absence of lithium. Since the salt segregates itself tothe PEO channels, the microstructure is now clearly visible and it ishighly suggestive of a lamellar morphology.

Measurements of the thickness of the lithium salt and poly(ethyleneoxide) lamellae are shown in FIG. 10 for SEO (74-98) at a saltconcentration of r=0.085. The first column corresponds to the product ofthe periodicity observed in our images and the volume fraction ofethylene oxide in the copolymer. The second column is the thicknesses ofPEO lamellae measured from bright field images of neat SEO copolymersstained with RuO₄. The last three columns correspond to lamellaemeasurements taken on energy filtered TEM images of O, Li and F. Thefirst three columns are a measure of the PEO lamellae thickness, whilethe last two are a measure of the Li channels. It is clear from FIG. 10that the salt lamellae are thinner than the PEO channels for SEO (74-98)at a salt concentration of r=0.085. The size of the Li domains werenormalized by the ethylene oxide domains to determine the extent of thisconcentration effect as a function of the molecular weight of PEO,M_(PEO), and these results are plotted in FIG. 11. It is clear that thethinning of the lithium lamellae is more dramatic for polymers withhigher molecular weight.

FIG. 12 is a comparison of the lithium salt distribution and the ionicconductivity of the copolymer electrolytes. There is a clear inverserelationship between the extent to which the Li segregates away from theinterface and the ionic conductivity. It is concluded that byconstructing SEO copolymer/salt mixtures where M_(PEO) is high, a novellithium salt structure within the nanostructured copolymer, with Li saltconcentrated in a central portion of the ionically conductive polymerblock, is produced, which results in highly conductive materials.

Nanostructured polymer electrolytes with soft conducting channelsembedded in a hard insulating matrix are beneficial for applicationsthat require independent control over mechanical and electricalproperties. The data presented above indicate that network phases arenot necessary to obtain ionic conductivity, which allows for thepossibility of using a wide variety of morphologies for designingionically conducting polymers. No special processing is needed to createpercolating conducting pathways in these materials; they are formedentirely by quiescent self-assembly. This suppresses dendrite growth inLi-polymer batteries and provides a cost-effective solution forcombining high energy density with good cyclability.

Applications

The electrolytes of the present invention are useful as solid phaseelectrolytes for high energy density, high cycle life batteries that donot suffer from failures due to side reactions and dendrite growth onthe Li electrodes. In one aspect, the invention relates to a batterycell, a simple schematic diagram of which is provided in FIG. 13,comprising a lithium anode, a cathode, and a solid phase block copolymerelectrolyte in accordance with the present invention disposed betweenthe anode and cathode. The cell can be cycled without detrimentaldendrite growth on the anode.

The invention may also be used, for example, as a power source forelectronic media such as cell phones, media players (MP3, DVD, and thelike), laptops and computer peripherals, digital cameras, camcorders,and the like. It may also be beneficial for applications in electricvehicles and hybrid electric vehicles.

CONCLUSION

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. It should be noted that there are many alternative waysof implementing both the process and compositions of the presentinvention. Accordingly, the present embodiments are to be considered asillustrative and not restrictive, and the invention is not to be limitedto the details given herein, but may be modified within the scope andequivalents of the appended claims.

1. A polymer electrolyte, comprising: a linear block copolymer having, aLi-ion conductive linear polymer block with a molecular weight of atleast 5000 Daltons; a structural linear polymer block with an elasticmodulus in excess of 1×10⁷ Pa; and an ionic conductivity of at least1×10⁻⁵ Scm⁻¹.
 2. The electrolyte of claim 1, wherein the linear blockcopolymer is a diblock copolymer.
 3. The electrolyte of claim 2, whereinthe diblock copolymer is characterized by bicontinuous phases of theconductive polymer block and the structural polymer block.
 4. Theelectrolyte of claim 3, wherein the structural linear polymer blockcomprises a major phase of the block copolymer.
 5. The electrolyte ofclaim 4, wherein the major phase structural polymer block forms a rigidframework and the conductive polymer block forms nanostructuredionically conductive channels with the rigid framework.
 6. Theelectrolyte of claim 1, wherein the structural polymer has a molecularweight of at least 5000 Daltons.
 7. The electrolyte of claim 1, whereinthe structural polymer has an elastic modulus of from about 1×10⁷ Pa to3 GPa.
 8. The electrolyte of claim 1, wherein the conductive polymer hasan elastic modulus of no more than 1 MPa.
 9. The electrolyte of claim 3,wherein the conductive and structural polymers each have molecularweights of at least 15,000 Daltons.
 10. The electrolyte of claim 3,wherein the conductive and structural polymers each have molecularweights of at least 50,000 Daltons.
 11. The electrolyte of claim 1,wherein the structural polymer block is ionically non-conductive. 12.The electrolyte of claim 11, wherein the structural polymer blockcomprises polystyrene.
 13. The electrolyte of claim 1, wherein theconductive polymer block comprises poly(ethylene oxide).
 14. Theelectrolyte of claim 1, wherein the conductive polymer block furthercomprises a Li salt.
 15. The electrolyte of claim 14, wherein the Lisalt is concentrated in a central portion of the ionically conductivechannels.
 16. The electrolyte of claim 1, wherein the ionic conductivityof the block copolymer increases with increasing molecular weight of theconductive polymer block.
 17. The electrolyte of claim 1, wherein theionic conductivity is at least 1×10⁻⁴ Scm⁻¹
 18. The electrolyte of claim3, wherein the bicontinuous phases have a lamellar arrangement.
 19. Theelectrolyte of claim 1, wherein the block copolymer is a triblockcopolymer further comprising an additional linear polymer block.
 20. Theelectrolyte of claim 19, wherein the triblock copolymer is apolystyrene-block-polyisoprene-block poly(ethylene oxide) (S-I-EO)triblock copolymer.
 21. A method of making a polymer electrolyte,comprising: in an oxygen and moisture free environment, forming a linearblock copolymer having a Li-ion conductive linear polymer block with amolecular weight of at least 5000 Daltons and a structural linearpolymer block with an elastic modulus of at least 1×10⁷ Pa; andincorporating a Li salt into the linear block copolymer; wherein theresulting polymer electrolyte has a ionic conductivity of at least1×10⁻⁵ Scm⁻¹.
 22. The method of claim 21, wherein the linear blockcopolymer is a diblock copolymer.
 23. The method of claim 22, whereinthe method comprises: synthesizing the structural polymer block byliving anionic polymerization; adding a monomer of the conductivepolymer block and a cryptand catalyst to the structural polymer blockliving anionic polymerization mixture; allowing a diblockcopolymerization reaction to proceed; terminating the reaction;precipitating and freeze-drying the resulting diblock copolymer product;blending the freeze-dried diblock copolymer with the Li salt in a drysolution such that the Li salt is dissolved into the conductive polymerblock; and freeze-drying the polymer/salt solution.
 24. The method ofclaim 23, further comprising subjecting a portion of the freeze-driedpolymer/salt solution to heat and pressure to form a freestandingpolymer electrolyte film.
 25. The method of claim 24, wherein theconductive polymer block comprises poly(ethylene oxide).
 26. The methodof claim 25, wherein the structural polymer block comprises polystyrene.27. The method of claim 26, wherein the Li salt is LiN[SO₂CF₃]₂.
 28. Abattery cell, comprising: a Li anode; a cathode; and a solid phasepolymer electrolyte disposed between the anode and cathode, theelectrolyte, comprising: a linear block copolymer having, a Li-ionconductive linear polymer block with a molecular weight of at least 5000Daltons; a structural linear polymer block with an elastic modulus inexcess of 1×10⁷ Pa; and an ionic conductivity of at least 1×10⁻⁵ Scm⁻¹.29. The cell of claim 28, wherein the conductive polymer has an elasticmodulus of no more than 1 MPa.
 30. The cell of claim 29, wherein theconductive and structural polymers each have molecular weights of atleast 15,000 Daltons.
 31. The cell of claim 30, wherein the conductiveand structural polymers each have molecular weights of at least 50,000Daltons.
 32. The cell of claim 31, wherein the conductive polymer blockcomprises poly(ethylene oxide).
 33. The cell of claim 32, wherein thestructural polymer block comprises polystyrene.
 34. The cell of claim33, wherein the Li salt is LiN[SO₂CF₃]₂.
 35. The cell of claim 28,wherein the cell can be cycled at least 80 times without dendrite growthon the anode.
 36. The electrolyte of claim 19, wherein the additionallinear block copolymer is the same as one of the conductive linear blockcopolymers or the structural linear block copolymers.
 37. Theelectrolyte of claim 19, wherein the additional linear block copolymeris different from both the conductive linear block copolymers and thestructural linear block copolymers.